Software defined radio

Under Software Defined Radio ( SDR ) one summarizes concepts for high frequency - transmitters and - receivers together, where smaller or larger amounts of signal processing with software be realized. The analog component can be a straight-ahead receiver or a heterodyne receiver (superhet). Above all, selection and modulation / demodulation are implemented in an SDR by means of digital signal processing .

Experiment platform of a software defined radio, usable as transmitter and receiver from 1 MHz to 6 GHz

General

SDR receiver with antenna socket in the form of a USB stick

An SDR system performs a large part of the signal processing with a general-purpose computer , if necessary combined with dedicated hardware such as signal processors and / or FPGAs . Receiver bandwidths of a few 10 MHz can be achieved with universal computers such as PCs. Larger bandwidths and more complex processing algorithms require special processors such as signal processors or FPGAs. The essential property is that the different parameters of the radio system such as modulation, different bandwidths, temporal behavior and different channel coding methods can be implemented simply by changing the software.

SDR is used, among other things, in amateur radio , the military and mobile communications, but it is also increasingly used in civil applications such as digital radio receivers. The flexibility and implementation of different protocol changes in real time are particularly useful here. A good and clear example is the implementation of the base stations of cellular networks as SDR. These could thus be cost-effectively upgraded to new standards within a very short time.

The hardware of an SDR consists, as shown in the diagram opposite, of two different types of SDR, at least of a transmitter and receiver module , as well as an A / D and D / A converter and the software-based digital signal processing in between. The signal processing is usually complex in the sense that a signal path consists of a pair of two parallel real number sequences , which is also referred to as an I / Q signal .

functionality

Block diagrams of two SDR implementations

ideal

The simplest and ideal SDR receiver would consist of an analog-to-digital converter with an antenna. The data read out would then be processed by a digital computer immediately after the analog-digital conversion.

The ideal transmitter would look similar: a computer generates a digital data stream via a digital-to-analog converter and a downstream antenna sends it.

Functional principles of SDRs

Today's SDRs work according to one of three functional principles:

Direct digitization of the input signal

After analog processing using filters and preamplifiers or attenuators that is as economical as possible, the input signal is digitized directly.

According to the Nyquist theorem , the input signal for digitization must be sampled with at least twice the maximum useful frequency in order to be able to reconstruct the signal. There are now A / D converters with sampling frequencies of up to 3.6 GSPS (Giga-Samples Per Second) with 12-bit resolution. This enables reception ranges of up to 1500 MHz.

Digitization at the intermediate frequency level

The first stages of such a receiver differ little from a conventional heterodyne receiver . The analog filters are designed for the largest useful signal bandwidth used. This not only reduces the requirements for the large-signal immunity for further processing, it also enables the sampling frequency to be drastically reduced: With an intermediate frequency bandwidth of e.g. B. 10 kHz, a sampling frequency of a good 20 kHz ( subsampling ) is sufficient .

This concept is now widespread because a sufficiently powerful digital signal processor (DSP) is significantly cheaper than various crystal filters with the required bandwidths. The DSP can also take on other functions such as gain control and demodulation - with significantly better properties and more options than conventional analog technology.

Direct mixer based on the I / Q process

Direct mix receiver is a receiver concept in which the input signal is mixed directly with an oscillator signal of the same carrier frequency and thus demodulated. This is how an audion worked in the 1920s if you wanted to receive Morse code.

The problem with conventional direct mixers is the lack of image rejection; H. a sinusoidal signal 1 kHz below the oscillator frequency delivers exactly the same output signal as a sinusoidal signal 1 kHz above the oscillator frequency. An SDR solves this problem through complex signal processing, i. H. by calculating with real and imaginary parts, which are also referred to as I / Q signals . The I stands for in phase and the real part. Q stands for Quadrature and for the imaginary part of the signal.

For this purpose, two parallel mixer stages are used in the input section of the direct mixer receiver, the oscillator signals of which are phase-shifted by 90 °. Such oscillator signals can be generated very easily with digital technology. The output signals of the two mixers are digitized in parallel and then processed digitally, with the Hilbert transformation playing a central role.

In the end, the Hilbert transformation causes a frequency-dependent delay without influencing the signal amplitude, so that the signal is rotated by 90 ° in phase. A 1 kHz signal is delayed by 250 µs, a 10 kHz signal by 25 µs. At the end, two direct overlay signals with 0 ° phase shift and with 90 ° phase shift are available. You can switch between the two sidebands by adding or subtracting the two signals.

SDR concepts used in practice

The direct mixer concept in particular is very widespread because it requires the least amount of electronics. Practically every cell phone is a transceiver based on the SDR direct mixer concept. Many other radio applications also work this way, e.g. B. with the CC1100 component from Texas Instruments, which works conventionally on the transmit side, but as SDR on the receive side.

Many cheap shortwave radios from current Chinese production refer to their digital signal processing.

The more complex concepts such as digitization at the intermediate frequency level or direct sampling of the input signal also work according to the I / Q concept. But they do without as much of the analog signal processing as possible in order to be able to use the higher accuracy of digital processing.

Advantages and disadvantages

The great advantage of software-defined radios is the flexibility and low costs when expanding to new or changed transmission standards through software upgrades .

In production, however, the construction of transmitters and receivers specially tailored to the transmission process can be significantly more cost-effective, so that the cost of production must be weighed against the costs of future expansions and new developments.

A disadvantage in the application is the required computing power of the signal processors used and the associated high power consumption.

Areas of application

Cellular: cell phones (since the mid-1990s) such as smartphones and base stations
Military communication technology
Amateur radio
measuring technology
research

SDR in amateur radio

A number of different implementations of software defined radios have become established over the past few years. In addition to commercial applications (such as DVB-T sticks), there are also variants in the amateur radio sector. In addition, there is PC-based software such as GNU radio , which can be used to implement a software-defined radio .

WebSDR and Internet accessible SDR receivers

Since around 2005 there have also been SDR receivers, such as the WebSDR, which are freely accessible on the Internet . An SDR variant for iOS has also been released. Additional SDR receivers in a large number of countries that also receive amateur radio can also be used via the Internet.

HPSDR

Amateur radio with HPSDR modules

Receiver with spectrum in the 160 m band

HPSDR ( High Performance Software Defined Radio ) is a platform for radio amateurs to develop modular electronic assemblies in SDR technology.

The project has existed since 2005. It uses open source software and free hardware , the assemblies are sold by Tucson Amateur Packet Radio Corporation . As part of HPSDR, both receivers and transceivers are developed and documented.

HPSDR has become the leading platform for non-commercial SDR developments. Circuit diagrams, construction descriptions, firmware, source code and manuals can be downloaded from the project's website; there is also a wiki, a discussion group (e-mail reflector) and a Teamspeak functionality.

RTLSDR

The RTLSDR project uses normal DAB and DVB-T sticks based on the popular RTL2832U chipset from Realtek . To do this, an alternative driver must be installed that provides an interface for a number of programs, e.g. B. the decoding of ADS-B for the localization of radio transponders . For example, SDR # is used as software, but there is also an SDR-based implementation of DAB . The frequency range that can be received with RTLSDR differs greatly depending on the tuner installed and is between 700 kHz and 1.7 GHz

tuner	Frequency range
Elonics E4000	64-1700 MHz
Rafael Micro R820T	24-1850 MHz
Rafael Micro R820T2	700 kHz - 1864 MHz

PC sound card

With the help of suitable software, for example SpecLab, a PC sound card can be used as an SDR for the longest wave range, the maximum receiving frequency being half the maximum sampling frequency. With the help of a sound card that can sample 192 kHz, it is even possible to set the PC clock exactly according to the time signal transmitter DCF77 .

literature

Bodo J. Krink: SDR - Software Defined Radio for the radio amateur: This is how the new technology works. Verlag für Technik und Handwerk, Baden-Baden 2009. ISBN 978-3-88180-848-4
Jouko Vankka: Digital Synthesizers and Transmitters for Software Radio . 1st edition. Springer, 2005, ISBN 1-4020-3194-7 .
Albert Heuberger, Eberhard Gamm: Software Defined Radio - Systems for Telemetry . Structure and functionality from the antenna to the bit output. 1st edition. Springer, 2017, ISBN 978-3-662-53233-1 .